Many imaging arrays for infrared (IR) radiation, particularly those comprised of a Group II-VI semiconductor material such as mercury-cadmium-telluride (HgCdTe), are required to be cooled to cryogenic temperatures during operation. As a result, the imaging system is required to include a suitable cryogenic cooler assembly, and also baffling such as cold shields and/or warm shields for excluding stray thermal radiation from the imaging array.
As can be appreciated, for some low-cost applications the use of a cryogenically cooled imaging array may be undesirable.
An uncooled thermal capacitor detector is described in U.S. Pat. No. 4,058,729, "Pyroelectric Apparatus Including Effectively Intrinsic Semiconductor For Converting Radiant Energy Into Electric Energy". Other conventional uncooled devices are described in journal articles by A. Sher et al. (Appl. Phys. Lett. 32, 713 (1978), and Wilson and Cotton (Proc. IRIS Detector Specialty Group, 1985).
In general, these devices exhibit a largest responsivity if the semiconductor material is intrinsic, or nearly intrinsic. Because of the low level of semiconductor doping, the depletion layer is relatively large, on the order of tens of micrometers for silicon doped in the range of 10.sup.11 to 10.sup.12 atoms/cm.sup.3. This necessitates a thick semiconductor, which results in an undesirable high thermal capacitance. Furthermore, the capacitance that is sensed is the static capacitance, which is undesirable for imaging applications where the detector must be read out rapidly. Another drawback to these conventional detectors is that the device must be biased close to the flat-band, or inversion threshold, of the thermal capacitor. That is, device modeling shows that temperature coefficient is bias dependent, and usefully large only for voltages in a range of one hundred millivolts about the flat-band and inversion-threshold voltages.
Other known approaches for providing uncooled thermal imaging arrays rely on bolometers made from V.sub.2 O.sub.3, amorphous silicon, and pyroelectric materials. However, these materials have various incompatibilities with silicon processes. Furthermore, the reported thermal coefficient of V.sub.2 O.sub.3 is only 2%/K.